As a wireless access scheme of a digital portable telephone system, a PHS system, and the like, a TDMA (Time Division Multiple Access)/TDD (Time Division Duplex) scheme in which TDMA and TDD are combined has been adopted. Recently, an OFDMA (Orthogonal Frequency Division Multiplexing Access) scheme using OFDMA based on a technique of OFDM (Orthogonal Frequency Division Multiplexing) has been proposed.
The OFDM is a scheme of dividing a carrier for data modulation into a plurality of “subcarriers” (subdivided carriers) orthogonal to each other and distributing and transmitting a data signal in each subcarrier.
Hereinafter, the overview of the OFDM scheme will be described.
FIG. 8 is a block diagram showing a configuration of an OFDM modulation device to be used at a transmitting side. Transmission data is input to the OFDM modulation device. The transmission data is supplied to a serial/parallel conversion unit 201 and converted into data including a plurality of low-speed transmission symbols. That is, a plurality of low-speed digital signals are generated by dividing transmission information. Parallel data is supplied to an inverse fast Fourier transform (IFFT) unit 202.
The parallel data is allocated to each subcarrier configuring OFDM and mapped in a frequency domain. Here, each subcarrier is modulated by BPSK, QPSK, 16QAM, 64QAM and the like. The mapping data is transformed from frequency-domain transmission data to time-domain transmission data by performing on IFFT operation. Thereby, multicarrier modulation signals into which a plurality of subcarriers orthogonal to each other are modulated independently are generated. An output of the IFFT unit 202 is supplied to a guard interval adding unit 203.
As shown in FIG. 9, the guard interval adding unit 203 sets a rear part of an effective symbol of transmission data as a guard interval and adds its copy to a front part of an effective symbol period for every transmission symbol. A base-band signal obtained by the guard interval adding unit is supplied to an orthogonal modulation unit 204.
The orthogonal modulation unit 204 orthogonally modulates a base-band OFDM signal supplied from the guard interval adding unit 203 using a carrier signal supplied from a local oscillator 205 of the OFDM modulation device, and performs frequency conversion into an intermediate frequency (IF) signal or a radio frequency (RF) signal. That is, after frequency-converting the base-band signal into a desired transmission frequency band, the orthogonal modulation unit outputs it to a transmission path.
FIG. 10 is a block diagram showing a configuration of an OFDM demodulation device to be used at a receiving side. An OFDM signal generated by the OFDM modulation device of FIG. 8 is input to the OFDM demodulation device through a predetermined transmission path.
An OFDM reception signal input to the OFDM demodulation device is supplied to an orthogonal demodulation unit 211. The orthogonal demodulation unit 211 orthogonally demodulates the OFDM reception signal using a carrier signal supplied from a local oscillator 212 of the OFDM demodulation device, performs frequency conversion from an RF signal or an IF signal to a base-band signal, and obtains a base-band OFDM signal. The OFDM signal is supplied to a guard interval removing unit 213.
The guard interval removing unit 213 removes a signal added by the guard interval adding unit 203 of the OFDM modulation device according to a timing signal supplied from a symbol timing synchronizing unit (not shown). A signal obtained by the guard interval removing unit 203 is supplied to a fast Fourier transform (FFT) unit 214.
The FFT unit 214 performs transformation to frequency-domain reception data by performing an FFT operation on input time-domain reception data. Demapping is performed in the frequency domain and parallel data is generated for each subcarrier. Here, the demodulation to the modulation of BPSK, QPSK, 16QAM, 64QAM, etc. performed for each subcarrier is performed. Parallel data obtained by the FFT unit 214 is supplied to a parallel/serial conversion unit 215 and output as reception data.
The above-described OFDM is a scheme for dividing a carrier into a plurality of subcarriers. The OFDMA is a scheme for collecting and grouping a plurality of subcarriers among the subcarriers in the above-described OFDM and performing multiplex communication by allocating one or more groups to each user. Each group is called a subchannel. That is, each user performs communication using one or more subchannels allocated. According to a communication data amount, a propagation environment, and the like, subchannels are adaptively increased/decreased and allocated.
Next, an example of channel configuration of a communication system adopting the OFDMA scheme will be described.
Patent Document 1 describes a communication method based on asymmetric channels with different bandwidths. In the communication method, downstream line (downlink) communication is performed by a broadband channel, and upstream line (uplink) communication is performed by a narrowband channel.
FIG. 11 is a configuration of transmission control between a terminal device and a base station in Patent Document 1. An OFDMA scheme is applied as an access scheme and different time slots within one frame are used by time division in the upstream line and the downstream line.
A predetermined number of slots T1, T2, . . . , Tn (where n is an arbitrary integer) of the first half of one frame are slots of an uplink period Tu as slots to be used for upstream line transmission from the terminal device in the base station. A predetermined number of slots R1, R2, . . . , Rn (where n is an arbitrary integer) of the second half of one frame are slots of a downlink period Td as slots to be used for downstream line transmission from the base station to the terminal device. As described above, a frame in which the uplink period and the downlink period are different from each other (times of the upstream and downstream are different from each other and slots configuring the upstream and downstream are different from each other) is referred to as an up-down asymmetric frame.
FIG. 12 is an example of channel configuration in which data having the above-described frame configuration is transmitted wirelessly.
In this example, guard band parts B1 and B2 narrower than bandwidths of broadband channels CH1 to CH4 exist at an upper side and a lower side of an available frequency bond B0, B1 and B2 are arranged with narrowband channels CII5 and CII6 which are narrower than the broadband channels CH1 to CH4, respectively.
The narrowband channels CH5 and CH6 arranged in the guard band parts are used as dedicated communication channels for low-speed access in the upstream line (uplink), and only the uplink period Tu of the first half of the frame configuration shown in FIG. 11 is used for wireless transmission.
Patent Document 2 describes a communication method in which communication between a base station and a mobile station is performed by allocating a time slot to be used in each communication counterpart on the basis of a situation of a transmission waiting cell for each of the downstream line (downlink) and the upstream line (uplink), and a communication device adopting an OFDMA/TDD scheme for allocating a user channel according to a transmission/reception amount and QoS of each asymmetric channel.
FIG. 13 is a schematic diagram showing a configuration of a communication system of Patent Document 2. Communication adopting the OFDMA scheme is performed between a base station (BTS) and a mobile station (MS).
FIG. 14 is a schematic diagram showing the format of a frame to be used in a wireless communication device of Patent Document 2. As shown, a unit frame (1 frame) includes an access channel (Ach), a control channel (Cch) of an upstream direction, a control channel (Cch) of a downstream direction, a user channel (Uch) of the downstream direction, and a user channel (Uch) of the upstream direction.
The number of time slots including each, of the user channel of the downstream direction and the user channel of the upstream direction is not fixed, and a boundary position is determined on the basis of a user channel allocation result.    Patent Document 1: JP-A-2000-115834    Patent Document 2: JP-A-2000-236343